1. Field of the Invention
The present invention broadly relates to a ball bearing assembly required to operate at a high RPM while immersed in a cryogenic fluid maintained under elevated pressure. It particularly relates to a bearing ball separator or cage for use in such an assembly.
2. Prior Art of the Invention
Many of the bi-propellant rocket (reaction) engines currently in use today utilize liquid oxygen as the oxidizer and either hydrogen or kerosene for the fuel. Since the efficiency of the engine is a function of, among other things, chamber pressure, it is essential that propellants be supplied at a high pressure. For example, a turbine-driven liquid oxygen pump may be required to have a discharge pressure as high as 8000 psi or more. The present turbo pumps are designed such that the bearings are located in a region where the liquid oxygen pressure does not exceed about 1000 pounds per square inch (psi) since the component keeping the spherical balls separated, the bearing ball separator or cage, has a limited oxygen pressure compatibility. It must be appreciated that there are numerous materials which are compatible with liquid oxygen at ambient pressure. However, at elevated pressures the number of compatible materials decreases substantially. The highest pressure at which a component may be considered compatible in liquid oxygen is determined in accordance with the 10 kilogram-meter (kgm) requirement of NASA's Marshall Space Flight Center (MSFC) Specification 106. Broadly, this comprises taking a sample of the material of which the component is made and immersing it in liquid oxygen contained within a sealed chamber. The specimen is then subjected to an impact equivalent to a 10 kilogram object falling 1 meter. During this test, the pressure within the sealed chamber is monitored, any abrupt increase in pressure (indicating a reaction between the liquid oxygen and some constituent of the sample) indicates that the use of the component at that particular pressure would not be safe (not compatible). Twenty consecutive samples of the material from the same lot must pass this test for the material to be rated acceptable for use at the test pressure.
Current bearing cages are made from a laminated cylinder of glass fabric reinforced polytetrafluoroethylene (PTFE). The process results in glass fiber bundles or threads that are coated but not impregnated with the PTFE. As a result, the laminated cylinder is very porous and subject to considerable opportunities for contamination. Thus, the separator or cage is limited to use in regions of the turbo pump where the liquid oxygen pressures do not exceed 1000 pounds. This results in serious design problems for high pressure oxygen pumps where discharge pressures may be as high as 8000 psi and future pumps will be required to provide even greater discharge pressures.